Messages in this thread | | | From | Vlad Buslov <> | Subject | Re: [PATCH net] sched: fix infinite loop when creating tc filter | Date | Mon, 11 Oct 2021 16:42:59 +0300 |
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Hi Volodymyr,
On Sun 10 Oct 2021 at 09:55, Volodymyr Mytnyk <volodymyr.mytnyk@plvision.eu> wrote: > From: Volodymyr Mytnyk <vmytnyk@marvell.com> > > After running a specific set of commands tc will become unresponsive: > > $ ip link add dev DEV type veth > $ tc qdisc add dev DEV clsact > $ tc chain add dev DEV chain 0 ingress > $ tc filter del dev DEV ingress > $ tc filter add dev DEV ingress flower action pass > > When executing chain flush, the "chain->flushing" field is set > to true, which prevents insertion of new classifier instances. > It is unset in one place under two conditions: > > `refcnt - chain->action_refcnt == 0` and `!by_act`. > > Ignoring the by_act and action_refcnt arguments the `flushing procedure` > will be over when refcnt is 0. > > But if the chain is explicitly created (e.g. `tc chain add .. chain 0 ..`) > refcnt is set to 1 without any classifier instances. Thus the condition > is never met and the chain->flushing field is never cleared. > And because the default chain is `flushing` new classifiers cannot > be added. tc_new_tfilter is stuck in a loop trying to find a chain > where chain->flushing is false. > > By moving `chain->flushing = false` from __tcf_chain_put to the end > of tcf_chain_flush will avoid the condition and the field will always > be reset after the flush procedure. > > Fixes: 91052fa1c657 ("net: sched: protect chain->explicitly_created with block->lock")
Thanks for working on this!
> > Co-developed-by: Serhiy Boiko <serhiy.boiko@plvision.eu> > Signed-off-by: Serhiy Boiko <serhiy.boiko@plvision.eu> > Signed-off-by: Volodymyr Mytnyk <vmytnyk@marvell.com> > --- > net/sched/cls_api.c | 5 +++-- > 1 file changed, 3 insertions(+), 2 deletions(-) > > diff --git a/net/sched/cls_api.c b/net/sched/cls_api.c > index d73b5c5514a9..327594cce554 100644 > --- a/net/sched/cls_api.c > +++ b/net/sched/cls_api.c > @@ -563,8 +563,6 @@ static void __tcf_chain_put(struct tcf_chain *chain, bool by_act, > if (refcnt - chain->action_refcnt == 0 && !by_act) { > tc_chain_notify_delete(tmplt_ops, tmplt_priv, chain->index, > block, NULL, 0, 0, false); > - /* Last reference to chain, no need to lock. */ > - chain->flushing = false; > } > > if (refcnt == 0) > @@ -615,6 +613,9 @@ static void tcf_chain_flush(struct tcf_chain *chain, bool rtnl_held) > tcf_proto_put(tp, rtnl_held, NULL); > tp = tp_next; > } > + > + /* Last reference to chain, no need to lock. */
But after moving the code block here you can no longer guarantee that this is the last reference, right?
> + chain->flushing = false;
Resetting the flag here is probably correct for actual flush use-case (e.g. RTM_DELTFILTER message with prio==0), but can cause undesired side-effects for other users of tcf_chain_flush(). Consider following interaction between new filter creation and explicit chain deletion that also uses tcf_chanin_flush():
RTM_DELCHAIN RTM_NEWTFILTER + + | | | +----------v-----------+ | | | | | __tcf_block_find | | | | | +----------+-----------+ | | | | | +----------v------------+ | | | | | tcf_chain_get | | | | | +----------+------------+ | | +--------v--------+ | | | | | tcf_chain_flush | | | | | +--------+--------+ | | | | +----------v------------+ | | | | | tcf_chain_tp_find | | | | | +----------+------------+ | | | |tp==NULL | |chain->flushing==false | | | +---------------v----------------+ | | | | | tp_created = 1 | | | tcf_chain_tp_insert_unique | | | | | +---------------+----------------+ | | | | +---------------v-----------------+ | | | | |tcf_chain_put_explicitly_created | | | | | +---------------+-----------------+ | | | v v
In this example tc_new_tfilter() holds chain reference during flush. If flush finishes concurrently before the check for chain->flushing, the chain reference counter will not reach 0 (because new filter creation code will not back off and release the reference). In the described example tc_chain_notify_delete() will not be called which will confuse any userland code that expects to receive delete chain notification after sending RTM_DELCHAIN message.
With these considerations I can propose following approach to fix the issue:
1. Extend tcf_chain_flush() with additional boolean argument and only call it with 'true' value from tc_del_tfilter(). (or create helper function that calls tcf_chain_flush() and then resets chain->flushing flag)
2. Reset the 'flushing' flag when new argument is true.
3. Wrap the 'flushing' flag reset code in filter_chain_lock critical section.
> } > > static int tcf_block_setup(struct tcf_block *block,
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